CTLA-4, a transmembrane protein expressed transiently on activated T cells, downregulates T cell activation and promotes tolerance in experimental animals. However, neither the molecular basis for these effects nor the extent to which CTLA-4 influences the human immune system are known. Our preliminary findings indicate that CTLA-4 triggering blocks cell cycle progression and promotes anergy in human CD4+ T cells, in vitro. The goals of this project are to define the molecular basis for these effects and determine if CTLA-4 plays a role in regulating human T cell responses, in vivo. Four specific aims are proposed: 1) to determine the function of CTLA-4 in the induction and maintenance of anergy in adult human T cells; 2) to define the molecular basis for CTLA-4 signalling by introducing into human T cells selected mutant forms of CTLA-4 and a variety of signalling intermediates; 3) to determine the role of CTLA-4 in regulating the immune response to self and non-self antigens, in vivo, in immunocompetent SCID/Hu mice; and 4) to determine if administration of anti-CTLA-4 antibody can reverse tolerance to a tumor associated antigen, in vivo, under conditions that mimic those in human cancer. The results of these studies should not only allow us to determine if and how CTLA-4 affects tolerance induction and maintenance in humans but potentially provide the basis for an effective immunotherapeutic approach to cancer.